1. Field of the Invention
This invention relates to a parallel filtering circuit used for parallel filtration of a fluid.
2. Description of the Prior Art
With the recent remarkable development in various industrial fields, it is increasingly strongly demanded to supply fluids of highest possible purity as various kinds of industrial raw materials or to use such pure fluids in various industrial manufacturing processes. Also, a fluid filtering circuit used for filtration of such a fluid is required to be capable of removing contaminants with a highest possible reliability and to be capable of maintaining the contaminant removing capability for a long period of time. To meet the above demands, various kinds of filters of the so-called depth layer type have been developed and put into practical use. This filter of the so-called depth layer type arrests contaminants of large and small particle sizes in its different filtering layers according to the particle sizes of the contaminants contained in a fluid. Thus, the filter of the depth layer filtering type has such a very excellent capability of contaminant removal that it can arrest a large amount of contaminants contained in a fluid over a relatively long useful service life.
The filter of the depth layer type has, for example, a structure as shown in FIGS. 7 and 8. FIG. 7 is a top plan view of the filter of the depth layer type, and FIG. 8 is a partly sectional, front elevation view of the filter of the depth layer type. As shown in FIGS. 7 and 8, the filter of the depth layer type is comprised of a bobbin 31 provided with flanges on both ends respectively of a hollow cylindrical member formed with many small perforations 31a and defining a fluid passage thereinside, and wires 32 wound around the bobbin 31 with such a winding density distribution that the winding desity progressively increases from the outermost wire layer toward the innermost wire layer. When a fluid containing contaminants flows past the outermost layer and toward the innermost layer of the wires 32 as shown by the arrows in FIG. 8, larger contaminant particles are arrested by the outer wire layers, while smaller contaminant particles are arrested by the inner wire layers. Thus, the fluid is progressively purified by the successive wire layers, and the purified fluid finally flows out through the perforations 31a into the fluid passage difined in the cylindrical member of the bobbin 31. However, the filter of the depth layer filtering type has been defective in that, once the flow of a fluid through the filter is obstructed or blocked due to local clogging of the filtering layers by contaminants particles, the rate of flow of the fluid through the filter decrease sharply, and a fatal situation may occur depending on the service. Therefore, it has been necessary to pay careful attention to its practical application. The relation between the flow rate of a fluid flowing through a filter of the depth layer filtering type and the period of time of operation of the filter for removal of contaminants is as, for example, shown by a characteristic curve in FIG. 6. It will be seen in FIG. 6 that the flow rate of the fluid flowing though the filter decreases progressively, although slightly, from the time immediately after the filter starts its operation. Such a progressive decrease in the fluid flow rate results from the fact that particles of contaminants contained in the fluid and arrested by the filter progressively accumulate inside the filter to cause local clogging of the internal layers of the filter. This local clogging spreads progressively with time until a flow blocking point G is reached where the internal layers of the filter are substantially entirely clogged with the contaminant particles. After this flow blocking point G is reached, the fluid flows through slight gaps remaining in the internal layers of the filter or flows between the accumulating contaminant particles while breaking through the contaminant particles. As a result, the contaminant particles further accumulate to sharply decrease the flow rate of the fluid flowing through the filter.
Therefore, a serious trouble may occur when the above filter is used for filtration of, for example, lubricating oil lubricating bearing for a shaft. The trouble is that the bearings had already burnt and stuck to the shaft due to shortage of supplied lubricating oil when the operator, who found a sharp decrease in the flow rate of the lubricating oil after the flow blocking point G was reached, judged that the filter had to be replaced.